The Englishman has scored seven goals and provided six assists in a standout debut campaign in England's top flight, with significant links to Manchester United this summer.
Grealish is yet to register a single goal or assist since the restart, however.
“When I was playing, I would not want to be the most fouled player in the Premier League," Souness told Sky Sports.
“That means I'm holding on to the ball for far too long. It means I'm not seeing the picture early enough.
“Taking an extra touch when I don't need to, it means you're not seeing the picture quick enough.
“There's no doubt about it, he's ability, he's silky on the eye, but I'm yet to be convinced by him."